The invention presented herein relates to thermal print heads.
Thermal print heads representative of the prior art thermal print head structures use a ceramic substrate that is covered with a glaze on which a number of metallic conductors are deposited and usually arranged as individual heater traces with one or more of the conductors provided as common conductor traces. A resistive material is coated on the glaze and extends between the end portions of the heater traces and the common conductor(s). An electrically insulating, wear resistant layer is usually coated over the resistive material to provide a smooth, wear resistant surface which is contacted by thermally sensitive paper that is moved relative to the surface during operation. During operation, voltage pulses are selectively applied to the heater traces and the common conductor(s) causing current flow through a portion of the resistive layers generating heat. Heat diffuses through the wear resistant layer to cause an imaging reaction to occur at thermally sensitive paper presented to the layer. Considering a single heater of a print head of this representative prior art configuration, the direction of heat flow toward the paper is perpendicular to the direction of the electrical current flow through the resistive material making up the body of the heater. It is also noted that the percentage of surface contact made between a heater conductor trace and the body of the heater is small compared to the total surface area of the heater.
U.S. patents representative of the prior art thermal print head structures include U.S. Pat. No. 3,161,457--H. Schroeder et al; U.S. Pat. No. 3,354,817--R. S. Sakurai et al; U.S. Pat. No. 3,578,946--W. L. Colello; U.S. Pat. No. 4,096,510--Arai et al; U.S. Pat. No. 4,136,274--Shibata et al; and U.S. Pat. No. 4,138,605--Stapleton et al.
Wear resistant material, when used in prior art print head structures, must be electrically insulating or it will electrically short circuit the heater. Being electrically insulating, the wear resistant material does not provide thermal conduction to the degree that can be provided by a metal. Reduction of the thickness of the wear resistant layer to improve its thermal conduction reduces the lifetime of the head. Response time and energy efficiency are impaired if the wear resistant layer is made thick enough to provide adequate wear lifetime.
A study of prior art thermal print head structures showed the need for a thermal print head that has a rapid response, making it possible to print a given amount of information in a shorter period of time than is possible with presently known thermal print heads. The study also indicated the need to provide higher resolution than is economically feasible with existing prior art teachings to increase the quality of a thermally produced image. A thermal print head which would operate with significantly less energy than is required by presently known print heads was also seen to be desirable, since it would reduce the cost of the drive circuitry used with a thermal print head. A reduction in the energy required was also recognized as a means for providing savings in the heat sink arrangements that are used with a thermal print head. Need was also seen for a thermal print head that is more resistant to wear and abrasion by various thermally responsive sheet materials that may be used with thermal print heads to provide a thermal print head with a longer useful life. It was desirable that the foregoing needs be achieved by a thermal print head having a construction cost that is significantly lower than is possible using presently known structural arrangements.